Implement base type and interface validation fallback for Minimal APIs

[bernardbr]

Hello to all contributors and maintainers. I would like to thank you for the great work being done on this project.

Motivation and Context

The current automatic validation pipeline rigidly resolves validators by looking for the exact type of the endpoint parameter. This prevents the validation of DTOs implementing interfaces or inheriting from base classes when the validator is defined for the abstraction.

This PR introduces the WithBaseTypeValidations setting, enabling the lookup engine to traverse the type hierarchy if a specific validator for the concrete type is not found. This enhancement will be essential for utilizing this library in an important, large-scale project I am currently working on.

Changes Made

• Configuration: Added the UseBaseTypeValidations flag and the WithBaseTypeValidations() fluent method.
• Lookup Refactoring: The GetValidator method now performs a recursive search ordered by: Concrete Type, Base Classes, and Interfaces.
• Scope: This change focuses on Minimal APIs. I deliberately chose not to modify MVC behavior at this stage. If required, support can be added in a separate PR by introducing a specific configuration and injecting it into the MVC ActionFilter.
• Documentation: Added XML comments to explain the new polymorphic behavior.

Benefits

• Reuse: Allows global rules on interfaces or base classes without duplicating validation logic.
• Flexibility: Native support for architectures using polymorphism in commands and DTOs.
• Reliability: Ensures standardized contracts are validated according to project standards.